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US006445861B1

( 12 ) United States Patent CIO) Patent No.: US 6,445,861 B1

Shaw et al. ( 45 ) Date of Patent: Sep. 3, 2002

(54) SOL-GEL PROCESSING TO FORM DOPED
SOL-GEL MONOLITHS INSIDE HOLLOW
CORE OPTICAL FIBER AND SOL-GEL
CORE FIBER DEVICES MADE THEREBY

(75) Inventors: Harry C. Shaw, Bel Air; Melanie N.

Ott, Columbia, both of MD (US);
Michele V. Manuel, Gainesville, FL
(US)

(73) Assignee: The United States of America as

represented by the Administrator of
the National Aeronautics and Space
Administration, Washington, DC (US)

( * ) Notice: Subject to any disclaimer, the term of this

patent is extended or adjusted under 35
U.S.C. 154(b) by 32 days.

(21) Appl. No.: 09/640,654

(22) Filed: Aug. 18, 2000

(51) Int. Cl. 7 G02B 6/16

(52) U.S. Cl 385/123; 385/12; 385/24;

385/16; 65/395

(58) Field of Search 385/123, 122-128,

385/147, 12-24, 142; 65/384, 395, 390

(56) References Cited

U.S. PATENT DOCUMENTS
5,911,025 A * 6/1999 Garito et al 385/123 X

6,289,161 B1 * 9/2001 Schotz et al 385/142

OTHER PUBLICATIONS

“Measurement of Low Oxygen Concentrations by Phospho-
escence Lifetime Using Fiber Optic”, Campo, Perez, et al.,
IEEE 1998 Instrumentation and Measurement Conference.
“An Integrated Optical technology Based on Sol Gel Glasses
on Silicon: The Nodes Project”, Yeatman, 1995 SBMO/
IEEE MTT-S IMOC ’95 Conference Proceedings.
“Applications of sol-gel films in optical wavelength filters”,
Blue, Mauchline, Stewart, Electronics Letters, Mar. 3, 1994,
vol. 30, No. 5, p. 402.

“Detecting Alpha Radiation by Scintillation in Porous Mate-
rials”, Keillor, Burggraf, IEEE Transactions on Nuclear
Science, vol. 44, No. 5, Oct. 5, 1997, 1741-1746.

“A Distributed Fiber Optic Sensor Based on Cladding Fluo-
resence”, Lieberman, Blyler, Cohen, Journal of Lightwave
Technology, vol. 8, No. 2, Feb. 2, 1990, 212-220.

* cited by examiner

Primary Examiner — Phan Palmer

(57) ABSTRACT

A process of fabricating a fiber device includes providing a
hollow core fiber, and forming a sol-gel material inside the
hollow core fiber. The hollow core fiber is preferably an
optical fiber, and the sol-gel material is doped with a dopant.
Devices made in this manner includes a wide variety of
sensors.

44 Claims, 15 Drawing Sheets

Passive Single-Ended, Integrated sol-gel Fiber Optic Sensor

Detector

Gases, Liquids

Interaction between environment and doped sol-gel creates light
via emission from luminescent centers in the dopant and/or sol-gel

U.S. Patent

Sep. 3, 2002

Sheet 1 of 15

US 6,445,861 B1

TEDS Sol -gel core doped with
caicofiuor in hollow core fiber,
365 m irradiation

Passive Single-Ended, Integrated sol-gel Fiber Optic Sensor

U.S. Patent

Sep. 3, 2002

Sheet 2 of 15

US 6,445,861 B1

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Sheet 4 of 15

US 6,445,861 B1

Detector

Passive Single-Ended, Integrated soi-gd Fiber Optic Sensor

Interactlort between environment and doped soi-gd creates light via
emission from luminescent centers in the dopant and/or sol-gel.

Single or Mufti-Mode Fiber

Hollow Core Fiber

Sample flows freely
into microcavity
and completely
surrounds sol-gd' core

FIG • 4

Hollow core fiber
with sol-gel and cavity

U.S. Patent

Sep. 3, 2002

Sheet 5 of 15

US 6,445,861 B1

Excited Emission Single-Ended,, Integrated sol-gel Fiber Optic Sensor, sol-gel cavity

Interaction between environment and doped sol-gel creates light via
emission from luminescent centers in the dopant and/or sol-gel
The emitted light is shifted in wavelength from excitation wavelength.

Narrowband
or Broadband
Light; Source

Single or Multi-Mode Fiber

Hollow Core Fiber

SoFgeisfs} 4* dopant js)

Calcofluor doped sol-gel in
hollow core fiber Irradiated
with blackiight

Sample flows freely
into microcavity
and completely
surrounds sohge! core

U.S. Patent

Sep. 3, 2002

Sheet 6 of 15

US 6,445,861 B1

Passive Single-Ended, Integrated soi-get Fiber Optic Sensor with sot-gel jacket

Single or Multi-Node Fiber Hollow Core Fiber
\ \

interaction between environment and: doped sohgel creates light
via. emission from luminescent centers in the dopant and/or soFgef

Detector

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The fiber is stripped of its outer
coating and the sensor can ad
as an evanescent mode coupler
sensor and an intrinsic detector.

Sol-get substrate
encapsulating fiber

Sobgefs(s) +• dopant(s)

FIG, 6

U.S. Patent

Sep. 3, 2002

Sheet 7 of 15

US 6,445,861 B1

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U.S. Patent

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Sep. 3, 2002

Sheet 9 of 15

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coating and the sensor can act surrounds sol-gel core

as an evanescent mode coupler
sensor and an intrinsic detector.

FIG. 9

Passive In-Line, Integrated sol-gel Fiber Optic Sensor

U.S. Patent

Sep. 3, 2002

Sheet 10 of 15

US 6,445,861 B1

light from the source. The shift could be a polarization shift, a phase shift, a wavelength

shift or some other characteristic.

Passive In-Line, Integrated sol-gel Fiber Optic Sensor with sol-gel jacket

U.S. Patent

Sep. 3, 2002

Sheet 11 of 15

US 6,445,861 B1

The sensor element interacts with its environment and shifts a characteristic in the incident
light from the source. The shift could be a polarization shift, a phase shift, a wavelength
shift, transmitted power or some other characteristic.

Excited Emission In-Line, Integrated sol-gel Fiber Optic Sensor

U.S. Patent

Sep. 3, 2002

Sheet 12 of 15

US 6,445,861 B1

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light from the source. The shift could be a polarization shift, a phase shift, a wavelei
transmitted power or some other characteristic.

In this case the source is capable of exciting emission in the sensor element

FIG. 12

Distributed Radiation Sensor using array of
Passive In-Line, Integrated and sol-gel Fiber Optic Sensor

U.S. Patent

Sep. 3, 2002

Sheet 13 of 15

US 6,445,861 B1

FIG. 13

Integrated Fiber Optic Sol-gel electro-optic device

U.S. Patent

Sep. 3, 2002

Sheet 14 of 15

US 6,445,861 B1

FIG. 14

Integrated Fiber Optic Sol-gel device as a wavelength demultiplexer

U.S. Patent

Sep. 3, 2002 Sheet 15 of 15 US 6,445,861 B1

US 6,445,861 B1

SOL-GEL PROCESSING TO FORM DOPED
SOL-GEL MONOLITHS INSIDE HOLLOW
CORE OPTICAL FIBER AND SOL-GEL
CORE FIBER DEVICES MADE THEREBY

ORIGIN OF THE INVENTION 5

Joint invention by Government and small business/
university contractor employees.

The invention described herein was made in the perfor-
mance of work under a NASA contract and by an employee io
of the United States Government and is subject to Public
Law 96-517 (35 U.S.C. §200 et seq.). The contractor has not
elected to retain title to the invention.

TECHNICAL FIELD

The invention relates to the field of fiber devices, and in
particular, to integrated fiber optic sol-gel sensors and
related technology.

BACKGROUND ART

The preparation of single and multicomponent glasses 20
using sol gel processes has been known for about 50 years.

Sol gel glasses can be prepared with dopant to modify the
physical, electronic or optical properties of the material.
Such modifications can include index of refraction, dielec-

2,5

trie constant, optical transmission characteristics. Sol-gel
materials have been used in combination with optical fibers
for many applications, and there is significant R&D activity
in the area of combining sol-gel materials with optical fiber
for sensing applications. Examples of possible uses include
chemical sensing, stress monitoring, pressure sensing, and 30
temperature sensing, in the fields of biomedical monitoring
and smart structures, for example.

The fiber optic sensor market has the potential for tre-
mendous growth. To illustrate the potential for fiber optic 35
sensing technologies, consider, for example, chemical sens-
ing. Driven by their increased use in biomedical
applications, fiber optic chemical sensors may have
accounted for almost 60% of the total fiber optic sensor
market in 1998. This corresponds to a revenue of $540
million for all chemical sensors.

As a specific example, there are very sensitive methods
for the detection of phosphatases. The advantages of using
an optical fiber sensor in such an application include the
following: the volume of enzyme and substrate containing 45
fluid solution needed for analysis can be smaller than in the
other techniques, which is important because the substrates
are very expensive; the sensor itself can be very small; and
the sensor can be relatively inexpensive and therefore dis-
posable. 5Q

Fiber optic sensors are a rapidly g owing field in other
areas as well. Since fiber optics are lightweight, EMI
immune, and passive, they are excellent candidates for a
variety of newly emerging applications such as smart sen-
sors. Smart sensors are embedded in a structure, e.g. an 55
aircraft fuselage, and can allow for online real time health
monitoring of the structure.

Some publications relating to fiber sensors are listed
below:

1. “Measurement of Low Oxygen Concentrations by 60
Phosphoescence Lifetime Using Fiber Optic”, Campo,
Perez, et. al., IEEE 1998 Instrumentation and Measure-
ment Conference

2. “An Integrated Optical Technology Based on Sol Gel
Glasses on Silicon: The Nodes Project”, Yeatman, 1995 65
SBMO/IEEE MTT-S IMOC ’95 Conference Proceed-
ings

3. “Applications of sol-gel films in optical wavelength
filters”, Blue, Mauchline, Stewart, Electronics Letters,
3 rd Mar. 1994, Vol. 30, No. 5, pg 402

4. “Detecting Alpha Radiation by Scintillation in Porous
Materials”, Keillor, Burggraf, IEEE Transactions on
Nuclear Science, Vol 44, No. 5, Oct. 5, 1997,
1741-1746

5. “A Distributed Fiber Optic Sensor Based on Cladding
Fluoresence”, Lieberman, Blyler, Cohen, Journal of
Lightwave Technology, Vol. 8, No. 2, Feb. 2, 1990,
212-220

However, the existing technologies involving sol-gel fiber
optic sensors have involved evanescent coupling to the fiber
optic through sol-gels applied as an external media. Existing
sol-gel sensors either have sol-gel as a thin film or deposited
material along the outside of the optical fiber, or are in the
shape of monoliths with dopants deposited on the surface of
the monolith. Such optical seniors are usually engineered
either by coating the surface of the optical fiber, or by
attaching directly to the fiber, water-soluble systems con-
tained in porous membranes.

The thin film or deposited material types operate through
evanescent optical coupling by light being coupled from the
outside film or material down to the core of the optical fiber
such that the sensor information in the form of an optical
signal can be guided down the core of the fiber. However,
this method of coupling is disadvantageously optically lossy,
allowing very little of the light in the cladding to actually be
coupled for guidance in the core.

Another drawback is that the amount of sensor reagent is
proportional to the amount of bulk sol-gel material pro-
cessed due to the solubility limits of the sol-gel, and the
amplitude of the sensor signal is directly proportional to the
amount of sensor reagent. Therefore, the smaller the sol-gel
volume, the smaller the effective volume for the sensing. In
thin film applications, the thickness is disadvantageously
less than 1 micron, because surface coating limits the
thickness of the reagent/sol-gel solution.

In processes where sol-gel monoliths are fabricated, the
dopant material is deposited into the pores, of the sol-gel
material on the outside surface of the monolith. However, in
the prior art, when sol-gel samples are polymerized
successfully, they are subjected to high temperatures during
the process which can disadvantageously fatigue an optical
fiber. The deposition is done in this fashion due to the
processing temperature required in the prior art for poly-
merization (approximately 1000 degrees C.). Also,
disadvantageously, when the monolith is exposed to envi-
ronmental elements, the dopants tend to leach out.

Applicant realized that it would be advantageous to have
a fiber sol-gel sensor which overcame the above disadvan-
tages and drawbacks of the prior art. Applicant realized that
such a device would be a fiber having a sol-gel core.
However, successful polymerization of sol-gel monoliths
inside of a hollow core fiber such that the system becomes
a functional waveguide was not known.

Therefore, a need existed for a fiber device having a
sol-gel core, and a method for manufacturing same, which
overcame the drawbacks and disadvantages of the prior art.

STATEMENT OF INVENTION

The invention relates to a process of fabricating fiber
devices having a doped sol-gel core, a plurality of fiber
device products made by the process, and a plurality of
apparatus utilizing a fiber device product made according to
the process.

It is, therefore, a principal object of this invention to
provide sol-gel core fiber devices, and in particular, to

US 6,445,861 B1

provide an optical fiber sensor using sol-gel processing of
monoliths inside a hollow core fiber.

It is another object of the invention to provide methods for
producing the sol-gel core fiber devices.

These and other objects of the present invention are
accomplished by the invention disclosed herein.

According to an aspect of the invention, the characteris-
tics of sol-gel materials and the sensors made with these
materials according to the invention, provide a number of
advantages. These characteristics and advantages include
their rigidity, their chemical inertness, their high porosity,
that they are hydrophilic, their optical transparency, their
good dynamic range, and their ease of processing.

The rigidity provides resistance to mechanical deforma-
tion. The is chemical inertness provides low chemical inter-
action with the environment. The high porosity entraps
photometric reagents, for example, but leaves them exposed
to exogenic analytes, with minimal chemical interaction or
interference with the source and emitted light. The charac-
teristic of being hydrophilic provides an increased availabil-
ity of reagents. An improved dynamic range means that
simultaneous measurement of several analytes by
co-immobilized sensor reagents at different wavelengths is
possible.

Also, since sol-gel changes color in the presence of
certain chemicals, chemical monitoring is enhanced.

Further, using sol-gels eliminates the need for other
equipment, e.g., signal processors, other sensors, in certain
applications.

According to an aspect of the invention, a device is
produced having a solid core monolith structure.

According to an aspect of the invention, it is an object to
practice a process for manufacturing fiber optic sensors
using hollow core optical fiber waveguides and customiz-
able silica sol-gel cores.

According to an exemplary embodiment of the invention,
an optical fiber sensor with a solid, monolithic, sol-gel core
is produced. This distinguishes the technology from other
sol-gel sensors, simplifying the fabrication process and
offering the versatility of being able to vary the properties of
the sol-gel core for custom applications.

According to an aspect of the invention, it is an object to
practice a process of partially filling the hollow core of the
fiber with a solid monolithic sol-gel. The remainder of the
core can then be filled with the sample to be sensed.

In particular, according to an aspect of the invention, a
number of criteria for a successfully polymerized sol-gel
fiber sensor element are is met. These criteria include
producing a solid monolith sol-gel core continuous and free
of cracks, such that light can be propagated down the fiber
containing the monolith.

According to an aspect of the invention, great improve-
ments over existing technology are achieved, including the
ability to produce monoliths within a hollow core fiber. The
processing by which sol-gel materials are fabricated suc-
cessfully into monoliths inside of hollow core optical fiber
is unique.

According to an aspect of the invention, the use of hollow
core fiber filled with a sol-gel core, makes a variety of novel
applications possible, and improves applications of sol-gel
fiber sensors that already exist. In the past, optical sensors
were usually engineered either by coating the surface of the
optical fiber or by attaching directly to the fiber, water-
soluble systems contained in porous membranes. Surface
coating limits the thickness of reagent/sol-gel solution to

approximately 1 /urn. According to an aspect of the present
invention, the ability to increase the amount of the sensor
reagent is achieved through the use of a sol-gel core.
Increasing the amount of sensor reagent increases the ampli-
5 tude of the detection signal, for example.

According to an aspect of the invention, the fabrication of
an exemplary sol-gel sensor includes polymerizing at low
temperatures, for example, a maximum of approximately
100 degrees C., to form a monolith inside a hollow core
10 optical grade fiber. In the past, in most cases when sol-gel
samples are polymerized successfully they are subjected to
high temperatures, e.g., 1000 degrees C., during the process
which can disadvantageously fatigue an optical fiber.

According to an aspect of the invention, low temperature
15 processing of a sol-gel core to allow temperature sensitive
dopants for inclusion is achieved. This provides the ability
to customize core for dielectric, optical, semiconductor,
electronic properties or a combination of these properties.
2Q According to an aspect of the invention, a low tempera-
ture processed metal alkoxide monolith as a customized,
dopable core for a hollow core fiber is produced.

According to this aspect of the invention, near room
temperature processing permits a wide choice of dopants,
25 including biological and biochemical, for example.
Advantageously, according to this low-temperature process-
ing aspect of the invention, proteins and enzymes can be
encapsulated within the sol-gel matrix without any degra-
dation or decrease in enzymatic activity.

30 According to an aspect of the invention, a large challenge
of fabricating cores inside of hollow core optical fiber is
overcome, namely, shrinkage and cracking of the material as
it polymerizes.

According to an aspect of the invention, a disclosed
35 process combines chemistry and materials processing, such
that sol-gels are fabricated inside of a hollow core fiber
without cracking to achieve a solid core.

According to an aspect of the invention, sol-gel material
is successfully polymerized without cracks into a solid
40 monolith inside a hollow core fiber, such that it can propa-
gate an optical signal similar to that of a optical fiber
waveguide. In other words, advantageously, a solid mono-
lithic sol-gel core fiber is produced which is continuous and
free of cracks such that light can be propagated down the
45 fiber containing the monolith.

Advantageously, according to an aspect of the invention,
the ability to control the shrinkage and bulk density of the
sol-gel during the curing phase to customize the fully
5Q polymerized final monolith is achieved.

According to an aspect of the invention, the option of
coprocessing or post-processing dopants is provided.
Because some dopants may not be compatible with the
sol-gel formation process, they must be added post-process,
55 i.e., after the sol-gel has been formed, prior to injection into
the fiber.

According to an aspect of the invention, in an exemplary
process, dopants are added as part of the sol-gel matrix. The
doped sol-gel is the inserted as a complete system into a
60 hollow core fiber. Therefore, the dopants stay intact through
the material matrix. Also, the entire doped sol-gel material
stays protected by the surrounding hollow core fiber of the
finished device.

According to an aspect of the invention, advantages
65 derived from using sol-gels include a large pore density
which allows doping with significant levels of scintillators
without quenching by the sol-gel matrix, a large variety of

US 6,445,861 B1

scintillators and luminesce t materials can be used for
detection of neutral particles, charged particles and photons
over a wide range of energies, and nuclear, biological and
chemical sensors can be produced.

According to an aspect of the invention, device perfor-
mance is enhanced by increasing the amount of dopant
material due to the geometry of the cylinder shape. The
amount of reagent is proportional to the amount of bulk
sol-gel material processed due to the solubility limits of the
sol-gel. Therefore, the larger the sol-gel volume the larger
the effected volume for the sensing performance.

According to an aspect of the invention, the ability to
place relatively large concentrations of dopant, trapped in
the sol-gel matrix, resistant to leaching effects from solvents
is achieved.

According to an aspect of the invention, the sol-gel
provides a substrate for reactions and catalysis sites, and
becomes a platform for observing and controlling reaction
kinetics.

According to an aspect of the invention, Sol-gel is doped
with a material that can be exploited to use the monolith
element inside the hollow core fiber as an optical fiber
sensor.

According to an aspect of the invention, a series of
customized optical fibers based on different dopants in the
sol-gel core are fabricated.

According to an aspect of the invention, a process is used
to generate a class of fiber optic devices containing a core
that can be customized for a variety of photonic applications.

According to an aspect of the invention, a core made of
sol-gels can be doped with materials that are soluble in the
sol-gel formula such that a variety of sensors can be fabri-
cated by using different dopant materials.

According to an aspect of the invention, the same process
can be used to fabricate a class of integrated sol-gel fiber
optic devices capable of sensing and/or as acting as active or
passive optoelectronic devices, for example. By controlling
the dopants, a wide variety of devices can be fabricated.

According to an aspect of the invention, a variety of
additional material dopants are possible for other sensing or
communications applications.

According to an aspect of the invention, using the exem-
plary method described herein, it is possible to create a
whole class of fiber optic devices based on one basic design.
The basic design can be altered to serve many different
sensing needs. The way in which the design is altered is by
changing the selection of the sol-gel material to custom suit
the sensing application. The appropriate sol-gel material for
the sensor application would be the material whose proper-
ties cause it to change color to indicate the presence of the
chemical or condition being monitored, for example.

According to an aspect of the invention, a fiber optic cable
is filled with a sol-gel core. Fiber optic cable offers the
advantage of being able to withstand harsh environments.
By using a fiber optic cable with modifiable properties of the
sol-gel core, the sol-gel hollow core configuration can be
customized to operate in various sensing applications. The
invention allows for the practicality of having just one basic
sensor design which, with simple modifications, can cover a
multitude of sensing needs.

According to an aspect of the invention, a large base of
sensor applications can be covered by the technology with
associated significant commercial potential for such sensors.

According to an aspect of the invention, it is an object to
produce single ended or in-line structures. In a single-ended

structure, the doped sol-gel core is located in a region at one
end of an optical fiber. In an in-line structure, the doped
sol-gel core is located in a region with regular optical fiber
on either side.

5 According to an aspect of the invention, a sensor is
fabricated for use as a luminescent element.

According to an aspect of the invention, sensing of
compounds via luminescence of dopants in the sol-gel is
accomplished.

io According to an aspect of the invention, optical and
electro-optical devices, such as wavelength division
multiplexers, and other devices, as part of fiber optic and
optical networks can be produced.

According to an aspect of the invention, a family of
15 sensors is produced that can be custom doped for various
applications in the field of fiber optic sensors. Since fiber
optics are lightweight, EMI immune, and passive, they are
excellent candidates for a variety of newly emerging appli-
cations such as smart sensors. Smart sensors are embedded
20 in a structure (e.g. aircraft fuselage) and can allow for online
real time health monitoring of the structure.

According to an aspect of the invention, applications of
devices made according to the invention include chemical
sensing, stress monitoring, pressure sensing, and tempera-
25 ture sensing in the fields of biomedical monitoring and smart
structures, for example.

According to an aspect of the invention, a sensor for
passive sensing, e.g., chemi-luminescence sensing, or active
sensing, e.g., laser excited luminescence sensing, is
30 achieved.

According to an aspect of the invention, a sensor accord-
ing to an exemplary embodiment of the invention can be
combined with other forms of fiber optic sensing, including
strain, temperature, electromagnetic, vibration, acoustic, for
35 example.

According to an aspect of the invention, a device accord-
ing to an exemplary embodiment of the invention is useful
for optical communications and other optical signal process-
ing applications, such as wavelength division multiplexing,
40 optical bandpass and bandstop filtering, and amplification.

According to an aspect of the invention, an exemplary
embodiment of the invention takes advantage of faraday
effect by converting sol-gel to a faraday glass for magneto -
45 optical and electro -optical devices.

According to an aspect of the invention, doping with
scintillating halides for radiation detection of x-rays, gamma
rays, low energy electrons, protons, or alpha-particles, for
example, is accomplished.

50 According to an aspect of the invention, an exemplary
embodiment of the invention can be used as detector in a
countermeasure against laser attacks or laser surveillance
against troops, equipment, or C 3 1 infrastructure (Command,
Control, Communications, Intelligence), for example.

55 According to an aspect of the invention, an exemplary
embodiment of the invention has applications in arms con-
trol and monitoring.

According to an aspect of the invention, advantages of the
exemplary fiber optics devices include immunity to electro-
60 magnetic interference and jamming, radiation hardening
capabilities, the provision of sensing and communications
on the same medium, high speed and wide bandwidth, the
ability to provide secure point to point links, and lightweight
configuration, requiring low power and occupying very little
65 space, which can be used covertly.

According to an aspect of the invention, advantages of an
integrated fiber optic sol-gel sensor include speed because

US 6,445,861 B1

the electric dipole transitions can produce radiative decays
on the order of a few nanoseconds. Sub-nanosecond decays
can be achieved with materials such as BaF 2 via core-
valence transitions.

According to an aspect of the invention, an integrated
fiber optic sol-gel radiation sensor allows for fast detection
times and detection of fast radiative emissions. Because the
emission occurs in the waveguide, most of the photons can
be transmitted directly to a detector.

According to an aspect of the invention, chemical and
biological sensing is achieved. An exemplary process
according to the invention is highly compatible with sensi-
tive biological materials because of the benign temperatures
and conditions.

According to an aspect of the invention, applications such
as wavelength division multiplexing, optical bandpass and
bandstop filtering, and amplification can be achieved.

According to an aspect of the invention, an exemplary
embodiment takes advantage of faraday effect by converting
sol-gel to a faraday glass for magneto -optical and electro-
optical devices. Doping with scintillating halides for radia-
tion detection of x-rays gamma rays, low energy electrons,
protons, a-particles. Communications and sensing are real-
ized in the same fiber. Use as detector in a countermeasure
against laser attacks or laser surveillance against troops,
equipment, or C 3 1 infrastructure is envisioned.

According to an aspect of the invention, fluorescent
transitions of alkaline phosphatase reactions can be moni-
tored by the integrated fiber optic sol-gel sensor according to
an exemplary embodiment of the invention. Phosphatase
activity measurements are very important in cell biology and
medicine for example in the detection of cancer and bio-
chemical processes in cells. There are very sensitive meth-
ods for the detection of phosphatases. An advantage of an
optical fiber sensor is that the volume of enzyme and
substrate containing fluid solution needed for analysis can be
smaller than in the other techniques. This is important
because the substrates are very expensive. Also, the sensor
is very small and disposable.

According to an aspect of the invention, a sensor can be
manufactured as a miniature disposable element mechani-
cally connected to the optical fiber.

According to an aspect of the invention, fiber optics filled
with sol-gel can be used in a variety of industries. These
include automotive as sensors in engines and for “intelli-
gent” highways; communications to speed up transfer of
data and information; environmental for real-time monitor-
ing of toxic compound emissions; food processing, for
quality control of food constituents; manufacturing in con-
trol systems and sensors within extreme environments; and
in medicine for in vitro diagnostics of physiological
analytes, monitoring blood constituents, drug dosage/
concentrations, and other body chemistry, for example.

According to an aspect of the invention, once an optical
fiber element is fabricated successfully, the element can be
spliced to an appropriate fiber(s), e.g., multimode or single
mode fibers, or attached to an integrated optical substrate or
other photonic device.

According to an aspect of the invention, communications
and sensing are realized in the same fiber.

According to an aspect of the invention, Sol-gel material
is successfully polymerized without cracks into a solid
monolith, such that it can propagate an optical signal similar
to that of a optical fiber waveguide.

According to an aspect of the invention, the doped sol-gel
serves as both detection medium and waveguide.

Advantageously, scintillation in the waveguide confines the
emitted photons to the waveguide. Other sol-gel fiber optic
sensors depend upon evanescent coupling through the clad-
ding of the fiber.

5 According to an aspect of the invention, by fabricating a
doped core, the sensor element and the waveguide core are
the same, which allows light to be guided to the detection
equipment without evanescent coupling.

A process of producing a sensor element comprised of a
io hollow core fiber with sol gel material polymerized inside
will be described. Sol gel material is successfully polymer-
ized without cracks into a solid monolith, such that it can
propagate an optical signal similar to that of a optical fiber
waveguide, Sol gel is doped with a material that can be
15 exploited to use the monolith element inside the hollow core
fiber as an optical fiber sensor. A dopant material such as
calcofluor or fluorescein is an example of a dopant material
used for fabrication of a luminescent sensor element. Cri-
teria for a successfully polymerized sensor element: a solid
20 monolith continuous and free of cracks such that light can be
propagated down the fiber containing the monolith.

The exemplary method follows process steps in order
from a cleaning process, to a sol-gel solution production
process, to a polymerization process in succession. Alterna-
25 fives that can be used with the other process steps are
described, as long as only one alternative is used per
processing.

The chemicals used include: Tetraethyl orthosilicate
(TEOS), ethanol, nitric acid, deionized water, hydrochloric
30 acid, calcofluor (fluorescent material), fluorescein
(luminescent material), sodium chloride.

The equipment used includes: Parr Microreactor, CMA
Microdialysis pump, 100/140 micron hollow core optical
fiber, 10/125 micron optical fiber, Tygon™ tubing, cleaving
35 tools, splice tubes, single mode optical fiber, and fusion
splicer. Although Tygon™ tubing is specified, it is noted that
any tubing could be used so long as it does not react with the
sol-gel material and the sol-gel material will not stick to it.

An exemplary embodiment of a process of fabricating a

fiber device includes providing a hollow core fiber, and

forming a sol-gel material inside the hollow core fiber. The

hollow core fiber is preferably an optical fiber. The sol-gel

material is doped with a dopant either prior to formation

inside the fiber or thereafter.

According to an aspect of the invention, the dopant may
be at least one of a fluorescent material, e.g., calcofluor, or
a luminescent material, e.g., fluorescein.

According to an aspect of the invention, the hollow core
50 fiber is processed prior to forming the sol-gel material
therein. This processing of the hollow core fiber prior to
forming the sol-gel material therein may include cleaning
the hollow core fiber.

According to an aspect of the invention, the cleaning of
55 the hollow core fiber includes injecting at least one cleaning
chemical into the hollow core fiber. According to an exem-
plary embodiment, the following chemicals are injected in
the following order: optimal grade hexane; HPLC grade
isopropanol; deionized water; and optimal grade acetone.

60 According to an aspect of the invention, the hollow core
fiber is air-dried after the injecting of the optimal grade
acetone, preferably for approximately 24 hours.

According to an aspect of the invention, the forming a
sol-gel material inside the hollow core fiber may include
65 producing a doped sol-gel solution; injecting the doped
sol-gel solution into the hollow core fiber; and polymerizing
the sol-gel solution inside the hollow core fiber.

US 6,445,861 B1

According to an exemplary embodiment of the invention,
the step of producing a doped sol-gel solution may include
mixing 20 ml of TEOS, 20 ml of deionized water, 20 ml of
ethanol, 2.5 ml of Hydrochloric acid 0.1 N, and 3 mg
calcofluor or fluorescein, to form a solution; placing the
solution into a reaction chamber in a sealed chamber, for
approximately 15 minutes, heating to 100 degrees C. during
which solution is stirred; venting the reaction chamber by
opening a gas release valve 100% for approximately 20
minutes or until microreactor reaches 8 psig (pounds per
square inch gauge, where 0 psig~14 psi absolute) during
which the solution cools at room temperature through con-
duction of the microreactor, so that the temperature of the
solution is approximately 80 degrees C. when removed,
without using induced cooling; placing the solution into a
microdialysis syringe; and pumping the solution into a
Tygon™ tubing which holds a piece of cleaved hollow core
fiber.

According to another exemplary embodiment of the
invention, the step of producing a doped sol-gel solution
may include mixing 50 ml of TEOS, 2.5 ml of ethanol, 10
ml of Hydrochloric acid 0.1 N, and 3 mg calcofluor or
fluorescein to form a solution; placing the solution into a
reaction chamber in a sealed chamber, for approximately 60
min, heat to 100 degrees C. during which solution is stirred
and left at 100 degrees C. for the rest of the 60 min period;
venting the reaction chamber by opening a gas release valve
100% for long enough for the chamber to stabilize at 8 psig
(pounds per square inch gauge, where 0 psig~14 psi
absolute) during which the solution cools at room tempera-
ture through conduction of the microreactor, so that the
temperature of the solution is approximately 80 degrees C.
when removed, without using induced cooling; placing the
solution into a microdialysis syringe; and pumping the
solution into a Tygon™ tubing which holds a piece of
cleaved hollow core fiber.

According to another exemplary embodiment of the
invention, the step of producing a doped sol-gel solution
may include mixing 20 ml of TEOS, 10 ml of deionized
water, 10 ml of ethanol, 2.5 ml of Hydrochloric acid 0.1 N,
and 3 mg calcofluor or fluorescein to form a solution;
placing the solution into a reaction chamber in a sealed
chamber, for 15 minutes, heat to 100 degrees C. during
which solution is stirred; venting the reactor by opening the
gas release valve 100% for approximately 20 minutes or
until microreactor reaches 8 psig (pounds per square inch
gauge, where 0 psig~14 psi absolute) during which the
solution cools at room temperature through conduction of
the microreactor, so that the temperature of the solution is
approximately 80 degrees C. when removed without
induced cooling; placing the solution into a microdialysis
syringe; and pumping the solution into a Tygon™ tubing
which holds a piece of cleaved hollow core fiber.

According to an exemplary embodiment of the invention,
the step of polymerizing of the sol-gel solution inside the
hollow core fiber may include, while a Tygon™ tubing
holding a hollow core fiber is injected with sol-gel material
via a micro dialysis pump method or by a dialysis pump
method, the other end of the tubing is inserted into a dialysis
bag 10 mm diameter, 150 mm long; the sol-gel material is
pumped in at 5 ml/minute into the Tygon™ tubing until a
few ml is dripping from the other side into the dialysis bag;
the rest of the Tygon™ tubing is placed into the bag and the
bag tied off on both ends; the bag containing the Tygon™
tubing with sol-gel material inside is placed in a deionized
water bath for 6 days; the bag is then removed from the
deionized water and placed in to a 10% saline solution for

3 hours; the tubing is then removed and allowed air dry for
four days; and the fiber element is then extracted from the
tubing.

According to an exemplary embodiment of the invention,
5 the step of polymerizing of the sol-gel solution inside the
hollow core fiber may include, while a Tygon™ tubing
holding hollow core fiber is injected with sol-gel material
via a micro dialysis pump method or by a dialysis pump
method, the other end of the tubing is inserted into a dialysis
10 bag 10 mm diameter, 150 mm long; the sol-gel is pumped in
at 5 ml/min into the Tygon™ tubing until the fiber element
is pushed out the other side of the Tygon™ tubing and
inserts into the dialysis bag; the bag is then pumped full of
sol-gel until the bag is completely full; the bag containing
15 the fiber element with sol-gel material inside is placed in a
deionized water bath for 6 days; the bag is then removed
from the deionized water and placed in to a 10% saline
solution for 3 hours; the fiber element is then removed from
the dialysis bag.

20 According to another aspect of the invention, a device
having a doped sol-gel core according to the present inven-
tion is subsequently encapsulated with a sol-gel material to
form a new device.

25 These and other aspects of the invention will become
apparent from the detailed description set forth below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates a sol-gel core fiber according to an
30 exemplary embodiment of the present invention.

FIG. 2 illustrates a Passive Single Ended, Integrated Sol
Gel Fiber Optic according to an exemplary embodiment of
the invention.

FIG. 3 illustrates Excited Emission Single Ended, Inte-
35 grated Sol Gel Fiber Optic Sensor according to an exem-
plary embodiment of the invention.

FIG. 4 illustrates Passive Single Ended, Integrated Sol
Gel Fiber Optic Sensor, sol gel/cavity according to an
40 exemplary embodiment of the invention.

FIG. 5 illustrates Excited Emission Single Ended, Inte-
grated Sol Gel Fiber Optic Sensor, sol gel/cavity according
to an exemplary embodiment of the invention.

FIG. 6 illustrates a Passive Single Ended, Integrated Sol
45 Gel Fiber Optic Sensor with sol gel jacket with a sol-gel
substrate encapsulating a sol-gel core fiber according to an
exemplary embodiment of the invention.

FIG. 7 illustrates Excited Emission Single Ended, Inte-
grated Sol Gel Fiber Optic Sensor with sol gel jacket
50 according to an exemplary embodiment of the invention.

FIG. 8 illustrates Passive Single Ended, Integrated sol-gel
Fiber Optic Sensor, sol-gel/cavity with sol-gel jacket accord-
ing to an exemplary embodiment of the invention.

55 FIG. 9 illustrates Excited Emission Single Ended, Inte-
grated sol-gel Fiber Optic Sensor, sol-gel/cavity with sol-gel
jacket according to an exemplary embodiment of the inven-
tion.

FIG. 10 illustrates Passive In-line, Integrated sol-gel Fiber
60 Optic Sensor according to an exemplary embodiment of the
invention.

FIG. 11 illustrates Passive In-line, Integrated sol-gel Fiber
Optic Sensor with sol gel jacket according to an exemplary
embodiment of the invention.

65 FIG. 12 illustrates Excited Emission In-line, Integrated
sol-gel Fiber Optic Sensor according to an exemplary
embodiment of the invention.

US 6,445,861 B1

FIG. 13 illustrates Distributed Radiation Sensor using
array of Passive In-Line, Integrated sol-gel Fiber Optic
Sensors according to an exemplary embodiment of the
invention.

FIG. 14 illustrates Integrated Fiber Optic Sol-gel electro- 5
optic device according to an exemplary embodiment of the
invention.

FIG. 15 illustrates Integrated fiber optic sol-gel device as
a wavelength demultiplexer. ^

DETAILED DESCRIPTION OF THE
INVENTION

The invention will now be described in more detail by
way of example with reference to the embodiment(s) shown ^
in the accompanying figures. It should be kept in mind that
the following described embodiment(s) is/are only presented
by way of example and should not be construed as limiting
the inventive concept to any particular physical configura-
tion. 20

An exemplary embodiment of a process of fabricating a
fiber device will now be described. The process includes
providing a hollow core fiber, and forming a sol-gel material
inside the hollow core fiber. The hollow core fiber is
preferably an optical fiber. The sol-gel material is doped 25
with a dopant either prior to formation inside the fiber or
thereafter. FIG. 1 illustrates a sol-gel core fiber made accord-
ing to an exemplary embodiment of the present invention.

In the case of producing a luminescent or fluorescent
sensor, the dopant would be at least one of a fluorescent 30
material, e.g., calcofluor, or a luminescent material, e.g.,
fluorescein.

The hollow core fiber is processed prior to forming the
sol-gel material therein. This processing of the hollow core
fiber prior to forming the sol-gel material therein includes 35
cleaning the hollow core fiber. The cleaning of the hollow
core fiber includes injecting at least one cleaning chemical
into the hollow core fiber. According to an exemplary
embodiment, the following chemicals are injected in the
following order: optimal grade hexane; HPLC grade isopro- 40
panol; deionized water; and optimal grade acetone.

The hollow core fiber is air-dried after the injecting of the
optimal grade acetone, preferably for approximately 24
hours.

A unique variation on this process is the use of organosi-
lane adhesion agents to improve adhesion of the sol gel to
the inner cavity of the fiber. The fiber is soaked in an
organosilane solution and cured in a vacuum oven prior to
the inclusion of sol gel. The organosilane compounds is 50
Silquest A-174 or equivalent. At least 0.2 ml of adhesion
agent is required. The fiber is soaked in the adhesion agent
for a minimum of 4 hours, removed from the agent and cured
for a minimum of 24 hours at a minimum temperature of 100

According to an exemplary embodiment of the invention,
the forming a sol-gel material inside the hollow core fiber
includes producing a doped sol-gel solution; injecting the
doped sol-gel solution into the hollow core fiber; and
polymerizing the sol-gel solution inside the hollow core go
fiber.

Three exemplary embodiments of the producing a doped
sol-gel solution will be described. A first embodiment
includes mixing 20 ml of TEOS, 20 ml of deionized water,

20 ml of ethanol, 2.5 ml of Hydrochloric acid 0.1 N, and 3 65
mg calcofluor or fluorescein, to form a solution; placing the
solution into a reaction chamber in a sealed chamber, for

approximately 15 minutes, heating to 100 degrees C. during
which solution is stirred; venting the reaction chamber by
opening a gas release valve 100% for approximately 20
minutes or until microreactor reaches 8 psig (pounds per
square inch gauge, where 0 psig~14 psi absolute) during
which the solution cools at room temperature through con-
duction of the microreactor, so that the temperature of the
solution is approximately 80 degrees C. when removed,
without using induced cooling; placing the solution into a
micro dialysis syringe; and pumping the solution into a
Tygon™ tubing which holds a piece of cleaved hollow core
fiber. Although Tygon™ tubing is specified, it is noted that
any tubing could be used so long as it does not react with the
sol-gel material and the sol-gel material will not stick to it.

A second embodiment of producing a doped sol-gel
solution includes mixing 50 ml of TEOS, 2.5 ml of ethanol,
10 ml of Hydrochloric acid 0.1 N, and 3 mg calcofluor or
fluorescein to form a solution; placing the solution into a
reaction chamber in a sealed chamber, for approximately 60
min, heat to 100 degrees C. during which solution is stirred
and left at 100 degrees C. for the rest of the 60 min period;
venting the reaction chamber by opening a gas release valve
100% for long enough for the chamber to stabilize at 8 psig
(pounds per square inch gauge, where 0 psig~14 psi
absolute) during which the solution cools at room tempera-
ture through conduction of the microreactor, so that the
temperature of the solution is approximately 80 degrees C.
when removed, without using induced cooling; placing the
solution into a microdialysis syringe; and pumping the
solution into a Tygon™ tubing which holds a piece of
cleaved hollow core fiber.

A third embodiment of producing a doped sol-gel solution
includes mixing 20 ml of TEOS, 10 ml of deionized water,
10 ml of ethanol, 2.5 ml of Hydrochloric acid 0.1 N, and 3
mg calcofluor or fluorescein to form a solution; placing the
solution into a reaction chamber in a sealed chamber, for 15
minutes, heat to 100 degrees C. during which solution is
stirred; venting the reactor by opening the gas release valve
100% for approximately 20 minutes or until microreactor
reaches 8 psig (pounds per square inch gauge, where 0
psig- 14 psi absolute) during which the solution cools at
room temperature through conduction of the microreactor,
so that the temperature of the solution is approximately 80
degrees C. when removed without induced cooling; placing
the solution into a microdialysis syringe; and pumping the
solution into a Tygon™ tubing which holds a piece of
cleaved hollow core fiber.

A fourth embodiment of producing a doped sol-gel solu-
tion includes addition of dimethyl form amide (DMF) as a
curing agent to the first, second and third embodiments. The
ratio of TEOS/DMF is not less than 100:1 and not greater
than 2:1.

A fifth embodiment of producing a doped sol-gel solution
includes addition of silica power as strengthening agent. The
proportion of silica to TEOS shall not exceed 500 mg silica
for every 20 ml TEOS

Two exemplary embodiments of the step of polymerizing
of the sol-gel solution inside the hollow core fibs will be
described. The first embodiment is as follows. While a
Tygon™ tubing holding a hollow core fiber is injected with
sol-gel material via a micro dialysis pump method or by a
dialysis pump method, the other end of the tubing is inserted
into a dialysis bag 10 mm diameter, 150 mm long; the
sol-gel material is pumped in at 5 ml/minute into the
Tygon™ tubing until a few ml is dripping from the other side
into the dialysis bag; the rest of the Tygon™ tubing is placed

US 6,445,861 B1

into the bag and the bag tied off on both ends; the bag
containing the Tygon™ tubing with sol-gel material inside is
placed in a deionized water bath for 6 days; the bag is then
removed from the deionized water and placed in to a 10%
saline solution for 3 hours; the tubing is then removed and
allowed air dry for four days; and the fiber element is then
extracted from the tubing.

A second exemplary embodiment of the step of polymer-
izing of the sol-gel solution inside the hollow core fiber is as
follows. While a Tygon™ tubing holding hollow core fiber
is injected with sol-gel material via a micro dialysis pump
method or by a dialysis pump method, the other end of the
tubing is inserted into a dialysis bag 10 mm diameter, 150
mm long; the sol-gel is pumped in at 5 ml/min into the
Tygon™ tubing until the fiber element is pushed out the
other side of the Tygon™ tubing and inserts into the dialysis
bag; the bag is then pumped full of sol-gel until the bag is
completely full; the bag containing the fiber element with
sol-gel material inside is placed in a deionized water bath for
6 days; the bag is then removed from the deionized water
and placed in to a 10% saline solution for 3 hours; the fiber
element is then removed from the dialysis bag.

According to the above-described process for manufac-
turing fiber optic sensors using hollow core optical fiber
waveguides and customizable silica sol-gel cores, a device
is produced having a solid, monolithic, sol-gel core. This
feature distinguishes the invention from other sol-gel sen-
sors which, as described in the background section above,
have significant drawbacks and limitations. Besides simpli-
fying the fabrication process, the present invention offers the
versatility of being able to vary the properties of the sol-gel
core for custom applications by varying the dopant.

As mentioned in summarizing the invention, a number of
criteria for a successfully polymerized sol-gel fiber sensor
element are met. These criteria include producing a solid
monolith sol-gel core continuous and free of cracks, such
that light can be propagated down the fiber containing the
monolith (see FIGS. 1 and 4, for example). Great improve-
ments over existing technology are achieved, including the
ability to produce monoliths within a hollow core fiber. The
processing by which sol-gel materials are fabricated suc-
cessfully into monoliths inside of hollow core optical fiber
described above, is unique.

The use of hollow core fiber filled with a sol-gel core,
makes a variety of novel applications possible, and improves
applications of sol-gel fiber sensors that already exist. In the
past optical sensors were engineered either by coating the
surface of the optical fiber or by attaching directly to the
fiber, water-soluble systems contained in porous mem-
branes. Surface coating limits the thickness of reagent/sol-
gel solution to approximately l_m. However, with the
present invention, the ability to increase the amount of the
sensor reagent is achieved through the use of a sol-gel core.
Increasing the amount of sensor reagent increases the ampli-
tude of the detection signal, for example.

The fabrication of an exemplary sol-gel sensor described
above includes polymerizing at low temperatures, for
example, a maximum of approximately 100 degrees C., to
form a monolith inside a hollow core optical grade fiber.
This is in contrast to the previous methods where, when
sol-gel samples are polymerized successfully, they are sub-
jected to high temperatures, e.g., 1000 degrees C., during the
process which can disadvantageously fatigue an optical
fiber.

Low temperature processing of a sol-gel core to allow
temperature sensitive dopants for inclusion is achieved with

the present invention. This provides the ability to customize
core for dielectric, optical, semiconductor, electronic prop-
erties or a combination of these properties. A low tempera-
ture processed metal alkoxide monolith as a customized,
5 dopable core for a hollow core fiber is produced according
to the invention. Near room temperature processing permits
a wide choice of dopants, including biological and
biochemical, for example. According to this low-
temperature processing aspect of the invention, proteins and
enzymes can be encapsulated within the sol-gel matrix
without any degradation or decrease in enzymatic activity.

A large challenge of fabricating cores inside of hollow
core optical fiber is overcome according to the present
invention, in particular, shrinkage and cracking of the mate-
rial as it polymerizes. The above -described process accord-
15 ing to an exemplary embodiment of the invention combines
chemistry and materials processing, such that sol-gels are
fabricated inside of a hollow core fiber without cracking to
achieve a solid core. The sol-gel material is successfully
polymerized without cracks into a solid monolith inside a
20 hollow core fiber, such that it can propagate an optical signal
similar to that of a optical fiber waveguide. The solid
monolithic sol-gel core fiber is continuous and free of cracks
such that light can be propagated down the fiber containing
the monolith.

25 The ability to control the shrinkage and bulk density of the

sol-gel during the curing phase to customize the fully
polymerized final monolith is achieved according to the
above described process.

The option of coprocessing or post-processing dopants is
30 provided. Because some dopants may not be compatible
with the sol-gel formation process, they must be added
post-process, i.e., after the sol-gel has been formed, prior to
injection into the fiber.

35 In an exemplary process according to the invention,
dopants are added as part of the sol-gel matrix. The doped
sol-gel is the inserted as a complete system into a hollow
core fiber. Therefore, the dopants stay intact through the
material matrix. Also, the entire doped sol-gel material stays
protected by the surrounding hollow core fiber of the fin-
ished device.

Advantages derived from using sol-gels include a large
pore density which allows doping with significant levels of
scintillators without quenching by the sol-gel matrix, a large
45 variety of scintillators and luminescent materials can be used
for detection of neutral particles, charged particles and
photons over a wide range of energies, and nuclear, biologi-
cal and chemical sensors can be produced.

Device performance is enhanced by increasing the
50 amount of dopant material due to the geometry of the
cylinder shape. The amount of reagent is proportional to the
amount of bulk sol-gel material processed due to the solu-
bility limits of the sol-gel. Therefore, the larger the sol-gel
volume the larger the effected volume for the sensing
55 performance.

The ability to place relatively large concentrations of
dopant, trapped in the sol-gel matrix, resistant to leaching
effects from solvents is achieved according to the invention.

The sol-gel provides a substrate for reactions and catalysis
60 sites, and becomes a platform for observing and controlling
reaction kinetics.

As described above, sol-gel is doped with a material that
can be exploited to use the monolith element inside the
hollow core fiber as an optical fiber sensor. A series of
65 customized optical fibers based on different dopants in the
sol-gel core can be fabricated, as would be apparent to one
skilled in the art.

US 6,445,861 B1

The exemplary process can be used to generate a class of
fiber optic devices containing a core that can be customized
for a variety of photonic applications. A core made of
sol-gels can be doped with materials that are soluble in the
sol-gel formula so that a variety of sensors can be fabricated
by using different dopant materials. The same process can be
used to fabricate a class of integrated sol-gel fiber optic
devices capable of sensing and/or as acting as active or
passive optoelectronic devices, for example. Simply by
controlling the dopants, a wide variety of devices can be
fabricated. A variety of additional material dopants are
possible for other sensing or communications applications.

The ability to jacket the fiber with sol-gels as well as
create intrinsic sol-gel core provides the ability to allow
multiple reactions to run while being sensed. For example,
the extrinsic sol-gel can be doped for chemi-luminescence of
a different reaction by-product than the intrinsic sol-gel.

Using the exemplary method described herein, it is pos-
sible to create a whole class of fiber optic devices based on
one basic design. The basic design can be altered to serve
many different sensing needs. The way in which the design
is altered is by changing the selection of the sol-gel material
to custom suit the sensing application. The appropriate
sol-gel material for the sensor application would be the
material whose properties cause it to change color to indi-
cate the presence of the chemical or condition being
monitored, for example.

According to the exemplary process described above, a
fiber optic cable is filled with a sol-gel core. Fiberoptic cable
offers the advantage of being able to withstand harsh envi-
ronments. By using a fiber optic cable with modifiable
properties of the sol-gel core, the sol-gel hollow core con-
figuration can be customized to operate in various sensing
applications. The invention allows for the practicality of
having just one basic sensor design which, with simple
modifications, can cover a multitude of sensing needs. A
large base of sensor applications can be covered by the
technology with associated significant commercial potential
for such sensors.

Single ended or in-line structures can be produced accord-
ing to the invention. FIG. 2 illustrates a passive single -
ended, integrated sol-gel fiber optic structure according to an
exemplary embodiment of the invention. FIG. 3 illustrates a
Excited Emission Single Ended, Integrated Sol Gel Fiber
Optic Sensor according to an exemplary embodiment of the
invention. In a single-ended structure, the doped sol-gel core
is located in a region at one end of an optical fiber. In an
in-line structure, the doped sol-gel core is located in a region
with regular optical fiber on either side.

FIG. 4 illustrates a Passive Single Ended, Integrated Sol
Gel Fiber Optic Sensor, sol gel/cavity according to an
exemplary embodiment of the invention. FIG. 5 illustrates
Excited Emission Single Ended, Integrated Sol Gel Fiber
Optic Sensor, sol gel/cavity according to an exemplary
embodiment of the invention. FIG. 6 illustrates a Passive
Single Ended, Integrated Sol Gel Fiber Optic Sensor with sol
gel jacket with a sol-gel substrate encapsulating a sol-gel
core fiber according to an exemplary embodiment of the
invention. FIG. 7 illustrates Excited Emission Single Ended,
Integrated Sol Gel Fiber Optic Sensor with sol gel jacket
according to an exemplary embodiment of the invention.
FIG. 8 illustrates Passive Single Ended, Integrated sol-gel
Fiber Optic Sensor, sol-gel/cavity with sol-gel jacket accord-
ing to an exemplary embodiment of the invention. FIG. 9
illustrates Excited Emission Single Ended, Integrated sol-gel
Fiber Optic Sensor, sol-gel cavity with sol-gel jacket accord-

ing to an exemplary embodiment of the invention. FIG. 10
illustrates Passive In-line, Integrated sol-gel Fiber Optic
Sensor according to an exemplary embodiment of the inven-
tion. FIG. 11 illustrates Passive In-line, Integrated sol-gel
5 Fiber Optic Sensor with sol gel jacket according to an
exemplary embodiment of the invention. FIG. 12 illustrates
Excited Emission In-line, Integrated sol-gel Fiber Optic
Sensor according to an exemplary embodiment of the inven-
tion. FIG. 13 illustrates Distributed Radiation Sensor using
10 array of Passive In-Line, Integrated sol-gel Fiber Optic
Sensors according to an exemplary embodiment of the
invention. FIG. 14 illustrates Integrated Fiber Optic Sol-gel
electro -optic device according to an exemplary embodiment
of the invention. FIG. 15 illustrates Integrated fiber optic
15 sol-gel device as a wavelength demultiplexer.

As described above, a sensor is fabricated for use as a
luminescent element. Sensing of compounds via lumines-
cence of dopants in the sol-gel is accomplished. Optical and
electro optical devices, such as wavelength division
20 multiplexers, and other devices, as part of fiber optic and
optical networks can be produced by the exemplary process
as well. A family of sensors can be produced, custom doped
for various applications in the field of fiber optic sensors. A
device according to an exemplary embodiment of the inven-
25 tion is useful for optical communications and other optical
signal processing applications, such as wavelength division
multiplexing, optical bandpass and bandstop filtering, and
amplification.

Since fiber optics are lightweight, EMI immune, and
30 passive, they are excellent candidates for a variety of newly
emerging applications such as smart sensors. Smart sensors
are embedded in a structure (e.g. aircraft fuselage) and can
allow for online real time health monitoring of the structure.
Other applications of devices made according to the inven-
35 tion include chemical sensing, stress monitoring, pressure
sensing, and temperature sensing in the fields of biomedical
monitoring and smart structures, for example. A sensor for
passive sensing, e.g., chemi-luminescence sensing, or active
sensing, e.g., laser excited luminescence sensing, can be
40 produced.

A sensor produced according to an exemplary embodi-
ment of the invention can be combined with other forms of
fiber optic sensing, including strain, temperature, electro-
45 magnetic vibration, acoustic, for example.

Another exemplary embodiment of the invention takes
advantage of the faraday effect by converting sol-gel to a
faraday glass for magneto-optical and electro -optical
devices.

50 In another embodiment of the invention, doping with
scintillating halides enables radiation detection of x-rays,
gamma rays, low energy electrons, protons, or alpha-
particles, for example.

Characteristic materials to be used in scintillating detec -
55 tors include:

Nal:Tl

CslrTl

CslrNa

60 Lu 2 Si0 5 :Ce
Y 2 Al 5 0 12 :Ce
Y 2 Si0 5 :Ce
ZnS:Ag
65 Nal
PbS0 4
Bi 4 Ge 3 0 12

US 6,445,861 B1

CdW0 4

K^LaC^iCe

BaCl 2

CdS:Te

Sol-gels will provide a stable substrate and integrating them
into a single system will provide a capable of radiation
detection over a wide range of energies along a single fiber
or a compact fiber bundle. The doping levels will typically
range from a 100 ppm to 1-10% as shown in FIG. 13.

Another exemplary application of the invention is use as
a detector in a countermeasure against laser attacks or laser
surveillance against troops, equipment, or C 3 1 infrastructure,
for example. The invention thus has applications in arms
control and monitoring.

Advantages of the exemplary fiber optics devices include
immunity to electromagnetic interference and jamming,
radiation hardening capabilities, the provision of sensing
and communications on the same medium, high speed and
wide bandwidth, the ability to provide secure point to point
links, and lightweight configuration, requiring low power
and occupying very little space, which can be used covertly.

Additional advantages of an integrated fiber optic sol-gel
sensor include speed because the electric dipole transitions
can produce radiative decays on the order of a few nano-
seconds. Sub -nanosecond decays can be achieved with
materials such as BaF 2 via core-valence transitions, for
example.

An integrated fiber optic sol-gel radiation sensor allows
for fast detection times and detection of fast radiative
emissions. Because the emission occurs in the waveguide,
most of the photons can be transmitted directly to a detector.

According to another exemplary embodiment of the
invention, chemical and biological sensing is achieved. The
exemplary process according to the invention is highly
compatible with sensitive biological materials because of the
benign temperatures and conditions.

Analysis of biochemical systems via chemi-luminescence
(CL) and bioluminescence (BL) is a major activity. The
integrated fiber optic sol gel sensor takes advantage of the
advances in this form analysis. Sol-gel can be doped with the
appropriate CL or BL reagent. The resulting fluorescence
takes place directly at the fiber optic interface for providing
for maximum signal transfer.

According to an embodiment of the invention, fluorescent
transitions of alkaline phosphatase reactions can be moni-
tored by the integrated fiber optic sol-gel sensor according to
an exemplary embodiment of the invention. Phosphatase
activity measurements are very important in cell biology and
medicine for example in the detection of cancer and bio-
chemical processes in cells. There are very sensitive meth-
ods for the detection of phosphatases. An advantage of an
optical fiber sensor is that the volume of enzyme and
substrate containing fluid solution needed for analysis can be
smaller than in the other techniques. This is important
because the substrates are very expensive. Also, the sensor
is very small and disposable. A sensor can be manufactured
as a miniature disposable element mechanically connected
to the optical fiber. The alkaline phosphatase reaction can be
monitored by doping the sol-gel with fluorescein
di-phosphate and adamantyl 1,2 dioxetane aryl phosphate as
two examples. FIGS. 2 and 4 are typical configurations of
sensor for this application.

Fiber optics filled with sol-gel can be used in a variety of
industries. These include automotive as sensors in engines
and for “intelligent” highways; communications to speed up
transfer of data and information; environmental for real-time
monitoring of toxic compound emissions; food for quality

control of food constituents; manufacturing in control sys-
tems and sensors, within extreme environments; and in
medicine in vitro diagnostics of physiological analytes,
monitoring blood constituents, drug dosage/concentrations,
5 and other body chemistry, for example.

Once an optical fiber element is fabricated successfully,
the element can be spliced to an appropriate fiber(s), e.g.,
multimode or single mode fibers, or attached to an integrated
optical substrate or other photonic device.
io Communications and sensing are realized in the same
fiber. Sol-gel material is successfully polymerized without
cracks into a solid monolith, such that it can propagate an
optical signal similar to that of a optical fiber waveguide.
The figures referred to above illustrate a crack-free monolith
15 inside a hollow core fiber according to an exemplary
embodiment of the invention.

The doped sol-gel serves as both detection medium and
waveguide. Scintillation in the waveguide confines the emit-
ted photons to the waveguide. Other sol-gel fiber optic
20 sensors disadvantageously depend upon evanescent cou-
pling through the cladding of the fiber. By fabricating a
doped core, the sensor element and the waveguide core are
the same, which allows light to be guided to the detection
equipment without evanescent coupling.

25 A sol-gel substrate encapsulating a sol-gel core fiber
device may be made according to an exemplary embodiment
of the invention. In this case, a doped sol-gel core containing
fiber, made according to the above-described methods, is
subsequently encapsulated in a doped sol-gel layer, so that
30 doped sol-gel is present inside and outside the fiber. FIG. 6
illustrates a Passive Single Ended, Integrated Sol Gel Fiber
Optic Sensor with sol gel jacket with a sol-gel substrate
encapsulating a sol-gel core fiber according to an exemplary
embodiment of the invention.

35 The table below cross references the types of sensors to
the appropriate configuration. The dopants have been pre-
viously described and new dopants are being tested on a
regular basis. There are thousands of possible dopant com-
binations.

Sensor

FIG.(s)

chemical sensor

2-12

fiber optic sensor

1-15

luminescent device

2-9

electro optical device

12, 14

biochemical sensor

2-12

radiation sensor

temperature sensor

biological sensor

2-12

laser-activated sensor

3, 5, 7, 9

acoustic sensor

10, 11, 12

electromagnetic sensor

10, 11, 12

Electric field sensor

7-12

optical device

14, 15

electro optical device

12, 14, 15

faraday effect

scintillating compound

2, 3, 6-13

laser detection device

3, 5, 7, 9, 13

fiber optic waveguide

2, 3, 6-15

particle detection device

phosphatase activity sensor

2-9

quality control of food constituents sensor

2-9

medical sensor

2-9

hazardous gas detector

2-12

integrated sensor reaction platform for
biochemistry and analytical chemistry

2-9

real-time monitoring of toxic compounds

2-9

sensor for real-time monitoring of
radiation emissions

US 6,445,861 B1

-continued

Sensor

FIG.(s)

single-ended device

2-9

in-line device

10-15

encapsulating sol-gel layer formed
on the outer surface of the fiber

6-9, 11

It will be apparent to one skilled in the art that the manner
of making and using the claimed invention has been
adequately disclosed in the above-written description of the
preferred embodiment(s) taken together with the drawings.

It will be understood that the above described preferred
embodiment(s) of the present invention are susceptible to
various modifications, changes, and adaptations, and the
same are intended to be comprehended within the meaning
and range of equivalents of the appended claims.

Further, although a number of equivalent components,
and/or process steps, may have been mentioned herein
which can be used in place of the components illustrated and
described with reference to the preferred embodiment(s),
this is not meant to be an exhaustive treatment of all the
possible equivalents, nor to limit the invention defined by
the claims to any particular equivalent or combination
thereof. A person skilled in the art would realize that there
may be other equivalent components presently known, or to
be developed, which could be used within the spirit and
scope of the invention defined by the claims.

What is claimed is:

1. A fiber device, comprising:

a hollow core fiber, said hollow core fiber having an inner
and outer surface; and

sol-gel material inside said hollow core fiber,

wherein the sol-gel material is doped with a dopant.

2. The fiber device according to claim 1, wherein the fiber
device comprises a fiber optic sensor.

3. The fiber device according to claim 2, wherein the fiber
optic sensor comprises a luminescent device when doped
with a luminescent material.

4. The fiber device according to claim 1, wherein the fiber
device comprises an electro optical device when doped with
E-0 active materials such as terbium-gallium.

5. The fiber device according to claim 1, wherein the fiber
device comprises a biochemical sensor when doped with
che mi-luminescence (CL) and bioluminescence (BL) mate-
rials.

6. The fiber device according to claim 1, wherein the fiber
device comprises a radiation sensor when doped with scin-
tillating compounds.

7. The fiber device according to claim 1, wherein the fiber
device comprises a temperature sensor when doped with
thermoluminescent materials.

8. The fiber device according to claim 1, wherein the fiber
device comprises a biological sensor when doped with
chemi-luminescence (CL) and bioluminescence (BL) mate-
rials.

9. The fiber device according to claim 1, wherein the fiber
device comprises a laser- activated sensor when doped with
chemi-luminescence (CL) or electroluminescent (EL) mate-
rials.

10. The fiber device according to claim 1, wherein the
fiber device comprises an acoustic sensor when doped with
acousto-optic materials.

11. The fiber device according to claim 1, wherein the
fiber device comprises an electromagnetic sensor when
doped as a faraday glass.

12. The fiber device according to claim 1, wherein the
fiber device comprises an electric field sensor when doped as
faraday glass and/or calcofluor.

13. The fiber device according to claim 1, wherein the

5 fiber device comprises an optical device.

14. The fiber device according to claim 13, wherein the
optical device comprises one of:

a wavelength division multiplexer/demultiplexer;

an optical bandpass filter;

an optical bandstop filter;

an optical switch;

an optical isolator; or

an optical amplifier.

15 15. The fiber device according to claim 1, wherein the

fiber device comprises an electro -optical device in combi-
nation with an electronic device.

16. The fiber device according to claim 1, wherein the
fiber device comprises a faraday effect device.

20 17. The fiber device according to claim 1, wherein the

fiber device dopant comprises a scintillating compound.

18. The fiber device according to claim 1, wherein the
fiber device comprises a laser detection device.

19. The fiber device according to claim 1, wherein the

25 fiber device comprises a fiber optic waveguide.

20. The fiber device according to claim 19, wherein the
fiber device further comprises a sensing device.

21. The fiber device according to claim 1, wherein the
fiber device dopant comprises a luminescent material.

30 22. The fiber device according to claim 1, wherein the

fiber device comprises a particle detection device.

23. The fiber device according to claim 1, wherein the
fiber device dopant comprises at least one protein.

24. The fiber device according to claim 1, wherein the

35 fiber device dopant comprises at least one enzyme.

25. The fiber device according to claim 1, wherein the
fiber device comprises a solvent-resistant device.

26. The fiber device according to claim 1, wherein the
fiber device comprises a phosphatase activity sensor.

40 27. The fiber device according to claim 1, wherein the

hollow core fiber has a diameter of approximately 10 /um .

28. The fiber device according to claim 1, wherein the
fiber device dopant comprises calcofluor.

29. The fiber device according to claim 1, wherein the

45 fiber device dopant comprises fluorescein or any member of

the class of fluoresceins.

30. The fiber device according to claim 1, wherein the
fiber device comprises a quality control of food constituents
sensor.

50 31. The fiber device according to claim 1, wherein the

fiber device comprises a medical sensor, for sensing at least
one of:

in vitro diagnostics of physiological analytes;

55 drug concentrations; and
other body chemistry.

32. The fiber device according to claim 1, wherein the
fiber device comprises a hazardous gas detector.

33. The fiber device according to claim 1, wherein the

60 fiber device comprises an integrated sensor reaction plat-
form for biochemistry and analytical chemistry.

34. The fiber device according to claim 1, wherein the
fiber device comprises a sensor for real-time monitoring of
toxic compounds.

65 35. The fiber device according to claim 1, wherein the

fiber device comprises a sensor for real-time monitoring of
radiation emissions.

US 6,445,861 B1

36. The fiber device according to claim 1, wherein the
fiber device comprises a single-ended device.

37. The fiber device according to claim 1, wherein the
fiber device comprises an in-line device.

38. The fiber device according to claim 1, wherein the 5
fiber device further comprises an encapsulating sol-gel layer
formed on the outer surface of the fiber.

39. The fiber device according to claim 1, further com-
prising means for providing a cavity between said inner
surface of said hollow fiber and said sol-gel material. 10

40. The fiber device according to claim 1, wherein a first
end of the fiber device is open ended.

41. The fiber device according to claim 1, further com-
prising sol-gel material on the outer surface said hollow core
fiber.

42. The fiber device according to claim 1, further com-
prising:

a first optical fiber operatively connected to a first end of
said hollow core fiber; and

a second optical fiber operatively connected to a second
end of said hollow core fiber.

43. The fiber device according to claim 42, further com-
prising sol-gel material coated on said outer surface of said
hollow core fiber.

44. The fiber device according to claim 42, further com-
prising an integrated circuit attached to said outer surface of
said hollow core fiber.

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Full text of "NASA Technical Reports Server (NTRS) 20080006988: Sol-gel processing to form doped sol-gel monoliths inside hollow core optical fiber and sol-gel core fiber devices made thereby"

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